Get rid of useless OpenMP related code

It seems that there was an intention to add BR2_ENABLE_OPENMP someday,
but it was in June 2007 (commit
c81807a9) and since then, nothing
occured. Therefore, get rid of this code, and just pass
--disable-openmp to gettext to keep the current behaviour.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>